Okay, so it’s not quite making the most of what you’ve got, but it’s the easiest way to sort your storage problem. If your phone accepts external storage, buy a micro SD memory card for it. That will give you an instant boost, and it’s a relatively cheap method of expanding your phone’s capacity.
Before you invest in a massive memory card though check your phone’s information for the highest capacity memory card it will accept. Some older phones will only support cards up 32 GB.
There are other options – using external devices or wireless drives – but part of the problem with relying on a memory card is that not all apps can be transferred to it. Some phones running Android 6.0 take the memory card and make it part of the system’s overall storage. It’s known as adoptable storage, and it means you can’t remove the card and use it in another device without formatting it, but it will be a cheap way to expand storage. You’ll need a card with a decent read/write speed too.